The invention relates to photo-electric detectors of the presence of an object, of the type comprising an electro-luminescent diode excited by an emitter circuit and coupled to a photo-electric receiver, a circuit for integrating the current delivered by the receiver, a thyristor switching means, controlled by the integrated current, a utilisation means and a means for feeding from the sector. It concerns more particularly the devices known as "two wire", in which the switching means is connected to the terminals of non-filtered rectified current of a diode bridge connected to the sector by the utilisation means. This latter is for example a relay controlled by the switching means. This type of detector, of which the industrial applications are numerous (for example, detection of the passage of an object in the luminous beam emitted by the electro-luminescent diode in the direction of the receiver, for the purpose of initiating a certain operation) has a range of several meters, greater than that of a detector with inductive receiver. The use, well known in such a circuit, of an electro-luminescent diode, permits one to do without employing a feed transformer, and thus to reduce the bulk and the consumption of electrical energy.
When it is desired, for purposes of simplification of the circuit, to construct it in the "two wire" version, it is necessary to face up to two known difficulties. The first is that, when the switching means is in the blocked state, the current consumed by the receiver assembly (which comprises the electro-luminescent diode, the receiver and the integrator circuit) passes through the utilisation means, as a result of which this current must be reduced as much as possible. The second difficulty is that, when the switching means is conductive, it is necessary to provide an auxiliary source of feed voltage for the receiver.
It is known, in order to overcome the first difficulty, to excite the electro-luminescent diode by means of a multivibrator comprising two transistors of complementary types which are simultaneously conductive during only a very small fraction of time.
An inconvenience of this known circuit is found in the fact that the consumption increases when the ambient temperature rises, by reason of the fact that the conduction period depends upon the gain of the transistors, which itself increases with temperature.